please give me the details in that formula, what is R?

43. The index model for stock A has been estimated with the following result:+ + RA= 0.01 +0.9 RM + ea. + + If om= 0.25 and Ra=0.25, the standard deviation of return of stock A is + + A. +0.2025. B. +0.2500.- which single-index model formula is used? C. +0.4500. D. 40.8100. R=b28M3; 0.25 = [(0.8120.25)2)/3; s= 0.4500